1. Field of Invention
The invention relates to a slippage detection system and method for use in a continuously variable transmission capable of continuously changing the gear ratio that is a ratio between an input rotation speed and an output rotation speed of the CVT.
2. Description of Related Art
As a continuously variable transmission capable of changing the gear ratio continuously, a belt-type continuously variable transmission and a toroidal type (traction-type) continuously variable transmission are known in the art. The belt-type continuously variable transmission is adapted to transmit torque and change the gear ratio by using a belt, and the toroidal type continuously variable transmission is adapted to transmit torque and change the gear ratio by using a power roller. In the belt-type continuously variable transmission, the belt is wound around drive and driven pulleys each capable of changing a groove width, and torque is transmitted by use of frictional force between contact surfaces of the pulleys and the belt. With this arrangement, the gear ratio of the CVT is changed by changing the groove width of the drive pulley so as to change an effective radius of the belt wound around the pulley.
In a toroidal continuously variable transmission, on the other hand, a power roller is sandwiched between an input disk and an output disk, and torque is transmitted by use of shearing force of traction oil present between the power roller and each of the disks. With this arrangement, the gear ratio of the CVT is changed by slanting or inclining the rotating power roller to thereby change the radius of the position at which torque is transmitted between the power roller and each disk. In the continuously variable transmission of the above types, a torque transmission portion takes the form of a surface, namely, torque is transmitted via surfaces of mutually facing members, so that the gear ratio can be continuously changed.
As a power transmitting mechanism that transmits torque via surfaces, a friction clutch, a friction brake, and the like are known. Such a friction clutch or a friction brake is constructed such that the entire areas of frictional surfaces come in contact with and are spaced apart from each other, with the frictional surfaces being designed in view of wears. A continuously variable transmission, on the other hand, is constructed so as to transmit torque by bringing a belt or a power roller into contact with a portion of a torque transmitting surface of each pulley or disk while continuously changing the torque transmitting portion. In such a continuously variable transmission, the torque transmitting surface is designed without substantially allowing for wear, and therefore a local wear of the torque transmitting surface may result in poor torque transmission or a damage to the continuously variable transmission.
Besides, there is a limit to the strength of constituent members or elements of the CVT, such as the belt, pulleys, disks, and traction oil. Therefore, the contact pressure between the corresponding members cannot be increased without limit to avoid slippage of the continuously variable transmission. Furthermore, when the contact pressure is increased to a certain level, the efficiency of power transmission and the durability of the continuously variable transmission may be undesirably reduced.
In continuously variable transmissions, therefore, the clamping force for clamping the belt or the power roller (or the load applied to clamp the belt or the power roller) is desired to be set to the minimum value in a range that ensures that excessive slippage (so-called macro-slip) does not occur between the belt and the corresponding pulley or between the power roller and the corresponding disk. Nevertheless, in general, the torque applied to the continuously variable transmission continuously changes. Especially when a vehicle in which a continuously variable transmission is used goes through a sudden acceleration or brake, or is brought into a complicated operating state, such as temporary idling or slippage of drive wheels, a sudden and temporarily large torque may be applied to the continuously variable transmission.
If the clamping force is set to be a greater value in preparation for such temporarily large torque, the power transmitting efficiency and the fuel efficiency may deteriorate while the vehicle is running in normal or steady-state conditions. Accordingly, it is preferable to perform a control to increase the clamping force or reduce the torque applied to the CVT when slippage due to large torque as described above is actually detected.
In the meantime, a system adapted for detecting a condition caused by slippage in a continuously variable transmission has been proposed in Japanese Laid-open Patent Publication No.62-2059. The system disclosed in this publication is arranged to determine a failure or problem in the continuously variable transmission. In this system, rotation speeds of a main pulley and a sub-pulley are measured using sensors to calculate a gear ratio. If the gear ratio thus measured or the rate of change in the gear ratio exhibits an extreme value that is not obtained in normal state, the system is determined to be at faulty.
The sensors used in the system disclosed in the above publication are the same as or equivalent to sensors generally used for a gear ratio control of a continuously variable transmission. With the above system thus constructed, therefore, a failure or a problem in the continuously variable transmission can be detected without using other sensor(s) newly provided for this purpose. Thus, the system disclosed in the above publication is designed for determining a failure of the continuously variable transmission, but is not provided, by nature, with any function of dealing with excessive slippage of the belt.
That is, the system disclosed in the above publication is constructed so as to detect a failure of the continuously variable transmission for the first time when the gear ratio or the rate of change in the gear ratio takes an abnormal value as a result of excessive slippage of the belt. The system, therefore, cannot be used for the purpose of avoiding a problem caused by excessive slippage of the belt. In other words, the system disclosed in the above publication is not able to detect, with sufficiently high speed and accuracy, the beginning of a so-called macro-slip (i.e., considerably large slip) of the belt or a state that may lead to a macro-slip. Consequently, the above-described conventional system cannot be used as a slippage detection system for performing a control for dealing with temporary slippage of the belt in the continuously variable transmission.